1. Field of the Invention
The present invention relates to a method for making lead pencils and colored pencils.
2. Description of the Prior Art
It is known that pencil leads are produced by pressing into leads a moist graphite-clay mixture in which the clay part is between 25 and 75%, depending on the desired hardness. These leads are dried and then solidified by firing at temperatures about 1100.degree. C. The firing is performed either using protective gas such as nitrogen or in graphite filling in order to prevent the graphite from burning in the leads. The writing properties of the leads thus produced may be improved by soaking the leads in binding and slip agents before encasing them in wood to increase resistance to breakage and improve handling.
It is also known that one can produce colored pencil leads whose colors would not tolerate burning by mixing synthetic materials such as cellulose derivates, e.g. carboxymethyl cellulose, nitrocellulose, ethyl and benzyl cellulose, with colors and slip additives and then pressing the mixture. Synthetic monomers such as styrene, methacrylic ester or thermoplastics such as polyvinyl chloride and polyvinyl acetate are also stated as binder phases for the colors.
The quality of the colored pencil leads with organic binding is far lower than the quality of fired pencil leads.
According to one proposal, pencil leads may be fired at lower firing temperature of less than 550.degree. C. if a mixture of graphite with enamel frit and quartz powder in a ratio of about 1:2 to 4:1 is used to make the leads. This is superior to the conventional production of pencil leads in that one can dispense with an inert gas atmosphere or a graphite filling, but one still requires a firing process with the necessary apparatus and fuel costs.